Hydrothermal synthesis of fibrous vanadate crystal in the system V2O5-H2O

Fibrous vanadate crystals of the system V2O5-H2O were synthesized hydrothermally at temperature ranging from 200 to 6000C under water vapor pressures of 50 to 400 atm for a duration within one month. A reagent grade V2O5 and its crystalline mass obtained by cooling after melting at 8000C for five hours were used as the starting materials. The products were examined by X-ray diffractometry, microscopic observation, infrared spectrometry, differential thermal analysis etc. A solubility curve of V2O5 in water at a constant pressure of 200 atm for two days showed that about 1.9g of V2O5 dissolved in water at 3200C and above 4000C its solubility decreased with increasing temperature. The length of the fibrous vanadate crystals increased with the treatment time below 3200C, but decreased above 4000C, at a constant pressure of 200 atm. Vanadate fiber length had a tendency to increase with the increase of temperatures and pressures in synthesis. The optimum conditions were found to be near 5000C and 200 atm for one hour treatment. The pH of the solutions in the reaction at 4000C and 200 atm for two days, affected the length of fiber. Long fibers were synthesized at the pH of 2.8, 4.2 and 6.0 after the reaction. X-ray powder diffraction patterns showed that V2O5.3H2O (navajoite) was stable under present experimental conditions and that above about 5000C V2O5.H2O and an unknown phase with platy crystal habit were produced with navajoite. DTA curves showed that a large peak for the endothermic reaction near 6100C was attributed to the evaporation of bond water. An absorption band near 3500 cm-1 on IR spectrum was assigned to the νsub( o h) or νsub(H2O) stretching vibrations and a band near 1000 cm-1 to the ν sub( v= o) stretching vibration. A silky fiber (navajoite) having a maximum length of 3.6 mm and 1 μm in diameter was synthesized. The fiber was elongated along [010], and found to have a light brown color and flexibility. (author)